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BGP
book

BGP

by Iljitsch van Beijnum
September 2002
Intermediate to advanced content levelIntermediate to advanced
288 pages
8h 36m
English
O'Reilly Media, Inc.
Content preview from BGP

Appendix C. Notes on the IPv4 Address Space

Up until 1993, address space was assigned using classful mechanisms: everybody needing 254 addresses or less got a Class C network, everybody needing 16384 addresses or less got a Class B network, and everyone needing more than 16384 got a Class A network. About half the Class A and two thirds of the Class B space were assigned this way. Use of the Class C space was limited at this time: only 192.0.0.0/8 and 193.0.0.0/8 were in use to some degree at that moment. Then, after the introduction of Classless Inter-Domain Routing, most new allocations to ISPs and assignments to end users were taken from Class C space, which consequently started to run out quickly around the turn of the millennium. Today, most new address space is allocated as classless blocks from Class A space. During the mid-1990s, most allocations were /19 or larger blocks, but this was changed to /20 later. Thus, prefixes longer than /20 anywhere other than in 192.0.0.0/7 are almost always the result of poor aggregation, which may or may not be for good reasons. Table C-1 shows an overview of the IPv4 address space.

Table C-1. Overview of the IPv4 address space

Address range

Class

0.0.0.0/1

A, classful networks with mask 255.0.0.0 (/8)

128.0.0.0/2

B, classful networks with mask 255.255.0.0 (/16)

192.0.0.0/3

C, classful networks with mask 255.255.255.0 (/24)

224.0.0.0/4

Class D range, meant for multicast use

240.0.0.0/4

Class E range, reserved for future applications

The Internet Assigned ...

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Publisher Resources

ISBN: 9780596002541Errata Page